In general, CDMA reception apparatuses are designed to allow a plurality of users to perform communication using the same frequency band. The respective users are identified by using spreading codes. In such mobile communication, since the transmission paths for the respective reception waves in multiplexed wave propagation vary in lengths, multiplexed waves with different propagation delay times interfere with each other.
Since a mobile station moves relative to base stations, signals through paths as propagation paths are subjected to Rayleigh variation in over-the-horizon communication. In CDMA communication, a plurality of multipath signals with different propagation delay times, which are subjected to Rayleigh variation, are collected to be in-phase combined (RAKE combined), thereby obtaining a diversity effect and improving reception characteristics.
Conventionally, there has been known a CDMA reception apparatus having a searcher section with a path control function for searching a finger section, a correlator group, an adder group, and post-addition correlation values for high-level reception timings and determining the reception timings at which signals should be received by the finger section.
In such a CDMA reception apparatus, when a path for a reception signal is to be detected, search paths from the searcher section are compared with tracking paths by a tracking means, backward protection is applied to detection of a coincidence of paths, forward protection is applied to detection of loss of a path, and the path capture state of a tracking path is classified into a complete loss state, backward protection state, complete protection state, or forward protection state, thereby obtaining a path state.
Conventional protection will be described with reference to the state transition diagram of protection paths in FIG. 7.
In backward protection, a path (reception timing) detected for the first time is protected as a path in a backward protection state 71 instead of immediately being set as a path in a synchronous state 73, and is determined as a path in the synchronous state 73 only after the path is kept detected at the same reception timings as a designated backward protection stage count 72.
In forward protection, if a path detected by previous processing cannot be detected by current processing, the path is protected as a path in a forward protection state 74 instead of immediately being deleted and setting an idle state 76, and is deleted from protection paths only after the path cannot be consecutively detected by a designated forward protection stage count 75.
As described above, it is necessary to perform protection so as not to frequently change the assignment of protection paths even with variations in level due to fading or slight changes in reception timing.
As an example of such techniques, “Spread Spectrum Communication Receiver” disclosed in Japanese Patent No. 2853705 is known.
This reference discloses a technique of improving demodulation characteristics even in the presence of a plurality of paths whose correlation levels are almost equal to noise level under multipath fading.
The above conventional CDMA reception apparatus and path protection method have the following drawback. Assume that all protection paths are paths in the forward protection state. When a state in which forward protection stage count cannot be detected continues, no protection path is deleted, and even a newly detected high-level path cannot be assigned.
In addition, the above conventional CDMA reception apparatus and path protection method have the following drawback. In sector changing operation in which the number of sectors to be used increases, a high-level path is newly detected at a high possibility. However, if no free space for a protection path is available, a detected path cannot be assigned until a free space is produced.
Furthermore, the conventional protection method has the following problem. A path set in the forward protection state is not deleted when a state in which forward protection stage count cannot be detected continues. For this reason, even when a mobile terminal moves out of the shadow of a building and waves that have not been detected can be directly detected, a detected path cannot be assigned until a free space for a protection path is produced.